This invention pertains to the casting of metals in sand molds, and particularly to methods and materials for increasing the lives of the mold patterns that are employed therein by incorporating graphite into the pattern coating composition.
The introduction of a molten metal into a cavity, or mold, where upon solidification, the resulting casting becomes an object whose shape was determined by the mold, is an old art. Equally as old is sand casting. In this molding process a wood, metal or plastic pattern is fabricated in the shape of the part to be produced. Sand is then compacted around the pattern in such a way that the top portion of the mold and the pattern can be removed, leaving a mold cavity in the shape of the pattern. Molten metal is then poured into the mold cavity.
It is well known that to increase the life of a mold and to make the removal of the casting easier, the surfaces of the mold cavity must be coated with a protective material. In the case of sand castings however it is the pattern which must be coated. Prior art coating compositions however deal primarily with mold coatings rather than pattern coatings.
Coatings for foundry cores and molds are basically mold release agents. They are used to obtain smoother casting surfaces with fewer defects. In its simplest form such a coating is simply a suspension of bentonite, kaolin and other members of the montmorillonite group of clays in water. As in the case of moldings, the use of sandcasting patterns is not without its own problems. The pattern surfaces erode and pit when successive mold cavities are produced using them. When such erosion occurs, molding sands have a greater tendency to adhere to the pattern when it is removed, affecting the mold cavity.
Commonly assigned U.S. patent application Ser. No. 10/072,638, now U.S. Pat. No. 6,960,367, issued Nov. 1, 2005, discloses a pattern coating composition comprising vegetable oil and a viscosity reducer such as mineral seal oil, alcohol or mixtures thereof.
Pattern release coatings in accordance with certain aspects of the present invention provide improved properties particularly when used in conjunction with green sand compositions in which the amount of seacoal has been reduced or completely eliminated. Seacoal is typically used in green sand to provide a variety of benefits. Seacoal develops lustrous carbon which results in a layer of carbonaceous material at the mold metal interface. Seacoal also reduces wall movement by developing into coking material during the metal casting process to form a cushion between the sand grains (Cushion Theory). Furthermore, seacoal contributes to the formation of an oolithic layer on the surface of sand grains. The oolithic layer provides various benefits including improving the flowability of molding sand and reducing roughness of the casting surface. Seacoal also contributes to a reducing atmosphere in the green sand mold during the metal casting process.
Efforts are underway to eliminate or reduce the amount of seacoal in green sand to reduce emissions associated with carbonaceous additives generated during the metal casting process. One method for replacing seacoal involves the addition of graphite into the green sand molding process by adding the graphite to the sand as a preblend or a separate addition. It is also known to rub graphite or plumbago on the surface of molds to enhance the release characteristic of castings. However, there is not sufficient time with modern high density molding equipment for an operator of a molding machine to rub plumbago on the mold. Accordingly, it would be desirable to have a method of providing lustrous carbon while eliminating or reducing the amount of seacoal in a manner that does not interfere with the molding process.
Pattern coating compositions in accordance certain aspects of the present invention provide many of the benefits associated with the use of seacoal in green sand while avoiding or reducing the problems associated with emissions from the seacoal.